Cytochrome P450s (P450) are a family of enzymes involved in the oxidative metabolism of both endogenous and exogenous compounds. P450 2D6 enzymes are widely distributed in the liver, intestines and other tissues (Krishna et al., Clinical Pharmacokinetics. 26:144-160, 1994). P450 2D6 enzymes catalyze the phase I reaction of drug metabolism, to generate metabolites for excretion. The classification of P450s is based on homology of the amino acid sequence (Slaughter et al The Annals of Pharmacotherapy 29:619-624, 1995). In mammals, there is over 55% homology of the amino acid sequence of CYP450 subfamilies. The differences in amino acid sequence constitute the basis for a classification of the superfamily of cytochrome P450 2D6 enzymes into families, subfamilies and isozymes.
Cytochrome P450 contains an iron cation and is a membrane bound enzyme that can carry out electron transfer and energy transfer. Cytochrome P450, when bound to carbon monoxide (CO), displays a maximum absorbance (peak) at 450 nm in the visible spectra, and is therefore called P450 (Omura et al., J. Biol. Chem. 239:2370, 1964).
Over 200 genes encoding cytochrome P450s have been identified, and are divided among over 30 gene families. These gene families are organized into subfamilies, which vary in regulation of gene expression and in amino acid sequence homology, substrate specificity, catalytic activity, and physiological role of the encoded enzymes.
The efficacy of a drug can be dramatically affected by its metabolism in the body. For drugs that are rapidly metabolized it can be difficult to maintain an effective therapeutic dose in the body, and the drug often must be given more frequently, in higher dose, and/or be administered in a sustained release formulation. Moreover, in the case of compounds for treating infectious disease, such as viral or bacterial infections, the inability to maintain an effective therapeutic dose can lead to the infectious agent becoming drug resistant. Many compounds that have strong biological efficacy and that would otherwise be potentially powerful therapeutics are rendered essentially useless by virtue of their short half-lives in vivo. A common pathway of metabolism for drugs is via oxidation by one or more cytochrome P450 2D6 enzymes. These enzymes metabolize a drug to a more polar derivative that is more readily excreted through the kidney or liver. First pass metabolism refers to the elimination of drugs via liver and intestinal CYP450 2D6 enzymes. First pass metabolism can lead to poor drug absorption from the GI tract due to extensive intestinal CYP450 metabolism, low plasma blood levels due to hepatic CYP450 metabolism, or both. Poor oral bioavailability due to CYP450 metabolism is a major reason for the failure of drugs candidates in clinical trials. In some instances, metabolic by-products of CYP450 2D6 enzymes are highly toxic and can result in severe side effects, cancer, and even death.
Some examples of drugs affected by CYP450 2D6 enzymes includes antidepressants (imipramine, clomipramine, desimpramine), antipsychotics (haloperidol, perphenazine, risperidone, thioridazine), beta blockers (carvedilol, S-metoprolol, propafenone, timolol), amphetamine, codeine, dextromethorphan, fluoxetine, S-mexiletine, phenacetin, propranolol.
Some examples of the effects of drug metabolism by CYP450 2D6 include
Dextromethorphan: CYP2D6 metabolizes dextromethrophan to dextrorphan. Individuals who express high levels of CYP2D6 (so-called rapid metabolizers) do not receive therapeutic benefits from dextromethorphan due to extensive first-pass metabolism and rapid systemic clearance.
Protease Inhibitors: Protease inhibitors and non-nucleoside reverse transcriptase inhibitors currently indicated for use in treatment of HIV or HCV are typically good substrates of cytochrome P450 2D6 enzymes; in particular, they are metabolized by CYP3A4 enzymes (see e.g. Sahai, AIDS 10 Suppl 1:S21-5, 1996) with possible participation by CYP2D6 enzymes (Kumar et al., J. Pharmacol. Exp. Ther. 277(1):423-31, 1996). Although protease inhibitors are reported to be inhibitors of CYP3A4, some non-nucleoside reverse transcriptase inhibitors, such as nevirapine and efavirenz, are inducers of CYP3A4 (see e.g. Murphy et al., Expert Opin Invest Drugs 5/9: 1183-99, 1996).
Human CYP450 isozymes are widely distributed among tissues and organs (Zhang et al., Drug Metabolism and Disposition. 27:804-809, 1999). With the exception of CYP1A1 and CYP2A13, most human CYP450 isozymes are located in the liver, but are expressed at different levels (Waziers J. Pharmacol. Exp. Ther. 253: 387, 1990). A solution to the problem of drug degradation and first-pass metabolism is to control the rate of drug metabolism. When the rates of drug absorption and metabolism reach a steady state, a maintenance dose can be delivered to achieve a desired drug concentration that is required for drug efficacy. Certain natural products have been shown to increase bioavailability of a drug. For example, the effect of grapefruit juice on drug pharmacokinetics is well known. See Edgar et al., Eur. J. Clin. Pharmacol. 42:313, (1992); Lee et al., Clin. Pharmacol. Ther. 59:62, (1996); Kane et al., Mayo Clinic Proc. 75:933, (2000). This effect of grapefruit juice is due to the presence of natural P450-inhibiting components. Other compounds also have been used for inhibition of P450. For example, the HIV-1 protease inhibitor Ritonavir® is now more commonly prescribed for use in combination with other, more effective, HIV protease inhibitors because of its ability to “boost” those other compounds by inhibiting P450-mediated degradation.